The present invention generally relates to a method for crystallization of an amorphous silicon film into a polysilicon film and more particularly, relates to a method for polysilicon crystallization from amorphous silicon by a simultaneous laser and rapid thermal annealing process.
There are numerous methods for converting an amorphous silicon into crystallized polysilicon has been developed and used. Excimer laser crystallization of amorphous silicon films is a promising method for fabricating polysilicon thin film transistor on glass substrate for application of active matrix liquid crystal displays. Excimer laser annealing has been shown to be superior to other crystallization techniques because of its high efficiency. In addition, it is a low temperature processing technique since the entire process takes place in only about 200 ns, consequently inducing a shallow heat-affected zone in the glass material. This interesting feature of excimer laser crystallization allows the use of inexpensive glass substrates instead of quartz substrates.
When a silicon film is deposited at a deposition temperature below 450xc2x0 C. using plasma enhanced chemical vapor deposition, the deposited film exhibits an amorphous structure. This can be confirmed by the fact that only a detraction pattern of defuse rings can be found in a transmission electron microscope image.
To convert an amorphous silicon to polysilicon, a subsequent annealing process after deposition can be used. The excimer laser process, furnace or rapid thermal annealing process, can be used to crystallize the amorphous film to polycrystalline.
While furnace annealing and laser annealing have been used to convert amorphous silicon to polysilicon, both techniques have severe drawbacks and limitations. For instance, in the conventional fabrication process for polysilicon thin film transistors, an excimer laser is used for annealing or crystallization of an amorphous silicon film. Since the excimer laser is of the pulse laser type, the period between pulses is only between 40 and 100 nano-seconds. Moreover, the energy of each pulse is in the low range of about 1 joule. As a result, the excimer laser is only effective in annealing a small area by scanning and a high level of overlapping is necessary to make up for the short period characteristics of the excimer laser and to obtain a satisfactory crystallization result. The conventional excimer laser therefore can only produce a low throughput for the crystallization process. Moreover, the variation in energy level between pulses of the excimer laser renders the process control difficult leading to a narrow process window that is available for the laser annealing method.
The conventional furnace annealing process, or the rapid thermal annealing (RTA) process, while producing high enough throughput for crystallization, the polycrystalline produced does not have the necessary high quality of large grain crystals, especially when compared to the grain size of crystals obtained by the laser annealing process.
It is therefore an object of the present invention to provide a method for polysilicon crystallization from amorphous silicon that does not have the drawbacks or shortcomings of the conventional thermal or laser annealing methods.
It is another object of the present invention to provide a method for polysilicon crystallization by the simultaneous annealing of laser energy and thermal energy.
It is another object of the present invention to provide a method for polysilicon crystallization by the simultaneous annealing of an excimer laser and the thermal energy from a Xe arc heating source.
It is another further object of the present invention to provide a method for polysilicon crystallization by moving a substrate on a conveyor under a combined energy source of laser and heat.
It is still another object of the present invention to provide a method for polysilicon crystallization by first merging an energy beam of laser and an energy beam of a thermal source and then irradiating the merged beam onto an amorphous silicon surface.
It is yet another object of the present invention to provide a method for polysilicon crystallization by using an excimer laser and a tubular Xe arc lamp simultaneous producing a merged energy beam on an amorphous silicon film.
In accordance with the present invention, a method for polysilicon crystallization by simultaneous laser and rapid thermal annealing is provided.
In a preferred embodiment, a method for polysilicon crystallization by simultaneous laser and rapid thermal annealing can be carried out by the operating steps of first providing a substrate that has an amorphous silicon layer on top; positioning the substrate on a conveyor situated inside a temperature-controlled chamber; providing a window that is substantially transparent to thermal and laser energy in a top wall of the temperature-controlled chamber; and directing a beam of thermal energy and simultaneously a beam of laser energy merging with the beam of thermal energy through the window onto a top surface of the substrate converting the amorphous silicon into polysilicon.
The method for polysilicon crystallization by simultaneous laser and rapid thermal annealing may further include the step of mounting a quartz window in the top wall of the temperature-controlled chamber, or the step of positioning a partially reflective lens in a path of the beam of thermal energy, or the step of positioning a partially transmissive lens in a path of the beam of thermal energy.
The method may further include the step of generating the beam of thermal energy by a radiation source selected from the group consisting of Xe arc lamp, Ar arc lamp, Kr arc lamp and W/halogen lamp. The method may further include the step of generating the beam of laser energy by an excimer laser source, or the step of generating the beam of laser energy by a XeCl laser source. The method may further include the step of producing the thermal energy by a tubular Xe arc lamp. The method may further include the step of reflecting incident beams of the thermal energy emitted by an arc lamp of a partially reflective lens prior to passing through the window, or the step of directing the beam of laser energy in a straight line through the partially reflective lens prior to passing through the window, or the step of reflecting thermal energy of a tubular Xe arc lamp by an oval-shaped mirror producing the incident beams.
The present invention is further directed to a method for crystallizing an amorphous silicon film into a polysilicon film by simultaneously irradiating with laser and thermal energy which can be carried out by the operating steps of first producing a laser beam and directing downwardly through a window into a process chamber; producing a thermal energy beam for reflecting downwardly by a partially reflective lens through the window merging with the laser beam; directing the merged laser and thermal energy beams onto an amorphous silicon film on a pre-processed substrate; and moving the pre-processed substrate linearly under the merged laser and thermal energy beams at a predetermined speed until the amorphous silicon film is converted to polysilicon film.
The method for crystallizing an amorphous silicon film into a polysilicon film by simultaneously irradiating with laser and thermal energy may further include the step of positioning a partially transmissive lens between a laser beam source and the pre-processed substrate for merging the laser beam with the thermal energy beam. The method may further include the step of positioning a partially reflective lens between a thermal energy source and the pre-processed substrate for merging the laser beam with the thermal energy beam. The partially reflective lens may be partially transmissive. The method may further include the step of preheating the process chamber to a temperature of at least 600xc2x0 C., or the step of moving the pre-processed substrate on a conveyor at a speed higher than 1 cm/min. The method may further include the step of producing the laser beam by an excimer laser source, such as an XeCl laser source. The method may further include the step of producing the thermal energy beam by a source selected from the group consisting of Xe arc lamp, Ar arc lamp, Kr arc lamp and W/halogen lamp.